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HTML

              
                <div class="theme-switch-wrapper">
  <label class="theme-switch" for="checkbox">
    <input type="checkbox" id="checkbox" />
    <div class="slider round"></div>
  </label>
</div>

<section>
  <article class="post">
    <h1>Lorem Ipsum</h1>
    <p class="post-meta">Neque porro quisquam est qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
      velit...</p>

    <p><strong>Lorem ipsum</strong> dolor sit amet consectetur, adipisicing elit. Suscipit voluptatum repellat
      praesentium impedit,
      earum cumque recusandae asperiores? <strong>Ullam</strong>, qui at. Ex aliquam ratione suscipit. Soluta
      voluptate consectetur quae est! Veniam.</p>

    <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Deleniti, architecto. Ducimus molestiae officia dolor
      voluptatem at, sequi debitis odit placeat rerum nisi quidem laudantium nobis in accusamus aut culpa obcaecati!
      Lorem ipsum dolor, <strong>sit amet consectetur</strong> adipisicing elit. Cupiditate provident, aliquid
      mollitia vero quisquam
      eligendi enim, eum incidunt dolorem ipsa voluptatem pariatur cum quis, repellat fugiat. In illo necessitatibus
      quasi!</p>
    <a href="#">Czytaj więcej</a>
  </article>
</section>
              
            
!

CSS

              
                @import url("https://fonts.googleapis.com/css?family=Lato:400,400italic,700|Sansita+One");

:root {
  --primary-color: #24242b;
  --secondary-color: #433055;
  --font-color: #3f3f3f;
  --bg-color: #f3f3f3;
  --heading-color: #2a2a23;
}

html[data-theme="dark"] {
  --primary-color: #f89898;
  --secondary-color: #8894b3;
  --font-color: #d8d8f6;
  --bg-color: #242424;
  --heading-color: #818cab;
}

body {
  font-family: "Lato", sans-serif;
  background-color: var(--bg-color);
  color: var(--font-color);
  max-width: 900px;
  margin: 0 auto;
  transition: background 200ms ease-in;
  padding: 0 30px;
  font-size: calc(1rem + 0.25vh);
}

h1 {
  color: var(--heading-color);
  font-family: "Lato", serif;
  font-size: 3rem;
  margin-bottom: 1vh;
}

p {
  font-size: 1.1rem;
  line-height: 1.6rem;
}

a {
  color: var(--primary-color);
  text-decoration: none;
  border-bottom: 3px solid transparent;
  padding-bottom: 5px;
  transition: border-bottom 1s;
  font-weight: bold;

  &:hover,
  &:focus {
    border-bottom: 3px solid currentColor;
  }
}

section {
  margin: 0 auto;
}

.post-meta {
  font-size: 1rem;
  font-style: italic;
  display: block;
  margin-bottom: 4vh;
  color: var(--secondary-color);
}

nav {
  display: flex;
  justify-content: flex-end;
  padding: 20px 0;
}

.theme-switch {
  display: inline-block;
  height: 34px;
  position: relative;
  width: 60px;

  input {
    display: none;
  }

  &-wrapper {
    display: flex;
    align-items: center;
    justify-content: flex-end;
    margin-top: 20px;

    em {
      margin-left: 10px;
      font-size: 1rem;
    }
  }
}

.slider {
  background-color: #ccc;
  bottom: 0;
  cursor: pointer;
  left: 0;
  position: absolute;
  right: 0;
  top: 0;
  transition: 0.4s;

  &:before {
    background-color: #fff;
    bottom: 4px;
    content: "";
    height: 26px;
    left: 4px;
    position: absolute;
    transition: 0.4s;
    width: 26px;
  }

  &.round {
    border-radius: 34px;

    &:before {
      border-radius: 50%;
    }
  }
}

input:checked {
  & + .slider {
    background-color: #66bb6a;

    &:before {
      transform: translateX(26px);
    }
  }
}

              
            
!

JS

              
                const toggleSwitch = document.querySelector(
  '.theme-switch input[type="checkbox"]'
);
const currentTheme = localStorage.getItem("theme");

if (currentTheme) {
  document.documentElement.setAttribute("data-theme", currentTheme);

  if (currentTheme === "dark") {
    toggleSwitch.checked = true;
  }
}

function switchTheme(event) {
  if (event.target.checked) {
    document.documentElement.setAttribute("data-theme", "dark");
    localStorage.setItem("theme", "dark");
  } else {
    document.documentElement.setAttribute("data-theme", "light");
    localStorage.setItem("theme", "light");
  }
}

toggleSwitch.addEventListener("change", switchTheme, false);

              
            
!
999px

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